This invention relates to a hydraulic control unit for controlling a speed change gear and a clutch which form an automotive transmission.
Transmissions for automotive vehicles include a manually operated type which can also be used as an automatic type with addition of a control unit which automatically controls the transmission. Such control unit generally has many component parts including a hydraulic actuator for effecting gear selection and gear shifting of the speed change gear in place of the manually operating lever of the speed change gear, a solenoid valve for controlling the hydraulic actuator, and a sensor means for sensing the selected position and shifted position of the speed change gear. A transmission with such a control unit has many connection pipes connecting component parts of the oil hydraulic circuit, e.g. the hydraulic actuator and the solenoid valve.
Furthermore, the control unit also includes a hydraulic actuator for driving the clutch of the automotive vehicle, which is incorporated in the same housing that accommodates the first-mentioned hydraulic actuator for selecting and shifting operations of the speed change gear. Therefore, the number of component parts of the control unit is large with the addition of a control or solenoid valve for controlling the hydraulic actuator for the clutch, a sensor for sensing the clutch position, etc., making the piping of the oil hydraulic circuit very complicated. This increases the assembly time required for manufacturing the transmission and degrades its operational reliability.
The aforementioned hydraulic actuator for the clutch includes a piston-cylinder type comprising a rod drivingly connected at one end to the clutch, a piston secured to the other end of the rod, and a cylinder in which the piston is slidably received and which cooperates with the piston to define a hydraulic oil chamber at one side of the piston and an air chamber at the opposite side thereof. In this type hydraulic actuator, the air chamber communicates with the atmosphere through an air breather opening in the air chamber at an upper location thereof. However, during movement of the piston within the cylinder barrel, hydraulic operating oil leaks bit by bit from the hydraulic oil chamber to the air chamber through the clearance between the cylinder and the piston and stays in the air chamber. This adversely affects the operation of a position sensor provided on the side of the air chamber, as well as the operation of the piston-cylinder type actuator. Moreover, as the air chamber contracts with movement of the piston to compress the air therein, operating oil staying in the air chamber is blown out of the air chamber through the air breather and scattered, thereby contaminating the peripheral parts.
The speed change gear is provided with an air breather opening for communicating the interior of the speed change gear casing with the atmosphere. The provision of two air breathers, however, is not only unfavorable costwise and functionally, but also reduces the efficiency of assembly and complicates the maintenance operation.